A positron emission tomography (PET) Buismuth Germinate Ortho (BGO) projection/tomographic imaging system was mechanically and electrically completed during this reporting period. The system is now undergoing additional refinements to improve imaging performance and user friendliness. Tests of the device to date have shown good agreement with predictions and the design goal of 2 mm spatial resolution in both projection and tomographic modes of operation has been achieved. The system is already being used in several mouse imaging projects to evaluate the effects of genetic changes on organ function.Phoswich scintillator arrays comprised of combinations of Lutetium Silicate Ortho (LSO), GSO, LGSO, or BGO were coupled to miniature metal-can position-sensitive photomultiplier tubes (PSPMT) in order to determine which combination of these scintillators might best form a depth-of-interaction detector module for PET imaging. Although good results were obtained with almost all combinations, practical reasons suggest that a dual layer phoswich consisting of GSO and LGSO will best serve this purpose. The scintillator layer of photon interaction in such a module can be easily discerned by long and short integration of the last dynode signal, while the position of the event within the transverse field-of-view can be determined by the PSPMT. Components, including custom-made preamplifier boards, are now being procured for a stationary, small diameter ring-type small animal PET scanner based on these GSO/LGSO modules.Progress continues to be made on an imaging probe system. A prototype probe with all necessary support electronics has been assembled and is now undergoing performance tests. Work was also continued to ascertain the imaging characteristics of a pair of continuous LSO slab scintillation cameras as candidates for a small animal imaging system.